This invention relates to coating compositions useful in preparing coated paper and other cellulosic materials, particularly to coating compositions thickened with a water-soluble polymer and to the coated articles prepared therefrom.
In the preparation of paper and other cellulosic web materials, e.g., paperboard, the paper is often coated with a pigment layer to improve the paper's opacity and to impart a smooth and receptive surface for printing. Conventionally, an aqueous suspension of a pigment such as kaolin clay, muscovite mica or calcium carbonate and a binder or adhesive for the clay particles such as starch or a synthetic polymer binder such as polyvinyl alcohol or a latex of a copolymer of styrene and butadiene is applied to the paper by means of high speed coating equipment such as a trailing blade coater. The water applied with the coating is subsequently removed from the coated paper sheet.
To facilitate subsequent printing of the coated paper, the coating is advantageously applied such that the pigment forms a smooth, level, ink-receptive layer which permits a uniform transfer of printing ink and imparts other desirable properties such as a high strength to permit subsequent printing without "picking." Unfortunately, at the high processing speeds economically desirable, the finished coating is often nonuniform as evidenced by streaking, mottling, strike through and a general nonuniformity of the coating weight along the length of the paper sheet.
Heretofore, to increase the coating speeds employed in coating paper and to improve the properties of the coated paper product, various additives have been incorporated in the coating composition. For example, a dispersing agent such as a polyphosphate helps transform the pigment particles into a uniform slurry thereby facilitating higher processing speed. Alternatively, various viscosity modifiers such as methyl cellulose and sodium alginate help control the flow properties of the coating color, thereby improving the smoothness of the pigment layer and other properties of the resulting coated paper. Increases in the processing speeds of the coating operation and improvements in the uniformity of the coating have also been accomplished by modifying the techniques and equipment used in the paper coating operations. While such additives and modifications have improved the properties of the finished paper products as well as permitting increased processing speeds, further uniformity of the pigment coating on the paper substrate is desired.